1. Field of the Invention
The present invention relates to piezoelectric ceramics and a piezoelectric ceramic device using the same, and particularly to piezoelectric ceramics useful as materials for piezoelectric ceramic devices such as a piezoelectric ceramic filter, piezoelectric ceramic oscillator, piezoelectric ceramic vibrator, etc, and a piezoelectric ceramic device using the ceramics.
2. Description of the Related Art
Piezoelectric ceramics comprising lead titanate zirconate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) as a main component are conventionally widely used as piezoelectric ceramics for piezoelectric ceramic devices such as a piezoelectric ceramic filter, piezoelectric ceramic oscillator, piezoelectric ceramic vibrator, etc. However, the piezoelectric ceramics comprising lead titanate zirconate as a main component has a composition containing a large amount of lead, and thus has the problem of deteriorating the uniformity of a product due to evaporation of lead oxides during the production process. In order to solve this problem, it is preferable to use a piezoelectric ceramic compact having a composition containing no lead or only a small amount of lead.
A piezoelectric ceramic compact comprising a bismuth layer compound such as SrBi4Ti4O15 as a main component has a composition containing no lead oxide, it does not cause the above problem.
However, a material used for a piezoelectric ceramic device, particularly a piezoelectric ceramic oscillator, generally preferably has a high maximum (Qmax) of the electrical quality factor Q (1/tan xcex4) in a band, i.e., at a frequency between the resonance frequency and anti-resonance frequency. A piezoelectric ceramic compact comprising the bismuth layer compound such as SrBi4Ti4O15 as the main component has the problem of failing to obtain a sufficient Qmax which enables practical use applications.
Accordingly, it is a main object of the present invention to provide a piezoelectric ceramic compact useful as a material for a piezoelectric ceramic device, comprising SrBi4Ti4O15 as a main component, containing no or a small amount of lead or lead compound, and exhibiting Qmax improved to a level permitting practical use, and a piezoelectric ceramic device using the piezoelectric ceramic compact.
In order to achieve the object of the present invention, there is provided a piezoelectric ceramic compact comprising a bismuth layer compound as a main component composed of Sr, Bi, Ti and oxygen, wherein when the molar ratio of Sr, Bi and Ti of the bismuth layer compound as the main component is a:b:c, the relations of 0.13xe2x89xa6a/c less than 0.25 and 3.5xe2x89xa6(2a+3b)/cxe2x89xa63.75 are satisfied.
The piezoelectric ceramic compact of the present invention may contain a bivalent metal element other than Sr or a trivalent metal element other than Bi at a ratio of about 0.05 mol or less (but not O) to 1 mol of Ti in the main component. The bivalent metal element other than Sr contained in the main component is at least one selected from, for example, Mg, Ca, Ba and Pb. The trivalent metal element other than Bi contained in the main component is at least one selected from, for example, Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er and Yb.
The piezoelectric ceramic compact of the present invention may further contain less than about 0.025 mol (but not O) of Zr relative to 1 mol of Ti in the main component.
The piezoelectric ceramic compact of the present invention may further contain Mn at a ratio of about 1.5% by weight or less (but not O) in terms of MnCO3.
A piezoelectric ceramic device of the present invention comprises a piezoelectric ceramic compact of the present invention, and an electrode formed on the piezoelectric ceramic compact.
Although the piezoelectric ceramic compact of the present invention comprises a sintered body obtained by burning a raw material composition for piezoelectric ceramics, the composition ratio is substantially constant before and after burning.
The piezoelectric ceramic compact of the present invention comprises a bismuth layer compound as a main component composed of Sr, Bi, Ti and oxygen, wherein when the molar ratio of Sr, Bi and Ti of the bismuth layer compound as the main component is a:b:c, the ratio is limited to a range so as to satisfy the relations of 0.13xe2x89xa6a/c less than 0.25 and 3.5xe2x89xa6(2a+3b)/cxe2x89xa63.75. This is because out of this range, a degree of Qmax permitting practical use applications cannot be obtained.
The piezoelectric ceramic compact of the present invention contains a bivalent metal element other than Sr or a trivalent metal element other than Bi at a ratio of about 0.05 mol or less (but not O) to 1 mol of Ti in the main component, thereby making the effect of the present invention significant. The ratio of the bivalent metal element other than Sr or the trivalent metal element other than Bi is about 0.05 mol or less (but not O) relative to 1 mol of Ti in the main component. This is because of the tendency that the addition of over about 0.05 mol of such a metal element deteriorates Qmax as compared with a case in which such a metal element is not added.
The piezoelectric ceramic compact of the present invention further contains less than about 0.025 mol (not O) of Zr relative to 1 mol of Ti in the main component, thereby making the effect of the present invention further significant. The ratio of Zr contained is set to less than about 0.25 mol to 1 mol of Ti in the main component because of the tendency that the addition of about 0.25 mol or more of Zr deteriorates Qmax as compared with the case in which Zr is not added.
The piezoelectric ceramic compact of the present invention may further contain Mn at a ratio of about 1.5% by weight or less (not O) in terms of MnCO3, thereby making the effect of the present invention further significant. The ratio of Mn contained is set to about 1.5% by weight or less in terms of MnCO3 because of the tendency that the addition of 1.5% by weight or more of Mn deteriorates Qmax as compared with a case in which Mn is not added.
The object, other objects, characteristics and advantages of the present invention will be made further clear by detailed description of embodiments of the present invention below.